If I remember right.. you pick the element you want when you use the ability. I seem to remember making icons for each element.
At least... I think so.
The effects should stack since they are temporary. But it may not always stack, like if you have a fire enchanted sword, the fire imbue might not work, I don't honestly remember, it's been a long time since I made the class. But the other elemental ones should.
Just play with it I guess. See what does and doesn't work together.